Insulating coatings and tubes of insulated electric wires and insulated cables that are used in wiring arranged inside electronic equipment and automobiles require high flame retardancy, high tensile properties, and a property that good tensile properties can be maintained even after exposure to high temperatures (heat resistance and heat-aging resistance).
In general, UL (Underwriters Laboratories Inc.) standards are used for evaluating the flame retardancy, heat resistance, and heat-aging resistance of electric wires. Regarding the flame retardancy, it is necessary to satisfy the Vertical-Specimen-Flame test (VW-1). Regarding heat resistance and heat-aging resistance, in UL standards, the heat resistance and heat-aging resistance are determined in terms of tensile strength retention and elongation retention after a heat-aging test. The heat-aging test is performed as follows. For example, in the case of a 150° C. rating, the test is performed in a Geer oven at 180° C. for seven days. In the case of a 125° C. rating, the test is performed in a Geer oven at 158° C. for seven days. In the case of a 105° C. rating, the test is performed in a Geer oven at 136° C. for seven days. In the 150° C. rating specified in UL 1581, the following characteristics are required: Regarding initial tensile properties, the elongation is 300% or more and the tensile strength is 13.79 MPa or more. In addition, even after the heat-aging test in a Geer oven at 180° C. for seven days, the tensile strength retention and the elongation retention are each 80% or more.
Hitherto, soft polyvinyl chloride compositions and halogen-based flame-retardant resin compositions prepared by incorporating a bromine-based or chlorine-based flame retardant in a polyolefin-based resin such as polyethylene, an ethylene-ethyl acrylate copolymer, or an ethylene-vinyl acetate copolymer have been used as materials that satisfy the above mechanical/physical properties and flame retardancy.
For example, Japanese Unexamined Patent Application Publication No. 5-81930 (PTL 1) discloses a resin composition containing, as a base polymer, a mixture of polyethylene having a melting point of 125° C. or higher and an ethylene-α-olefin copolymer and, as flame retardants, a metal hydroxide, a halogen-based flame retardant, and an organosilicon compound such as methacryloxypropyltrimethoxysilane. It is disclosed that an insulating coating layer produced by cross-linking a coating layer composed of this resin composition satisfies the flame retardancy specified by the Vertical-Specimen-Flame test and the requirements for the 125° C. rating (after the heat-aging test in a Geer oven at 158° C. for seven days) specified in UL standards.
Japanese Patent No. 3279206 (Japanese Unexamined Patent Application Publication No. 10-168248: PTL 2) discloses that a cross-linked insulating coating using a flame-retardant resin composition prepared by incorporating a halogen-based flame retardant and flowers of zinc (zinc oxide) in a polyolefin-based resin which is an ethylene-α-olefin copolymer and which has a density of less than 0.89 satisfies the 150° C. rating.
As described above, it is known that, in the case where a halogen-based flame retardant is used, a cross-linked insulating coating containing a polyolefin-based resin as a main component can satisfy the requirements for the 125° C. rating, and furthermore, the requirements for the 150° C. rating.
However, electric wires and cables produced by using any of these flame-retardant resin compositions cause a problem in that a hydrogen halide gas is generated during an incineration treatment. Therefore, recently, so-called non-halogen flame-retardant resin compositions, which contain no halogen-based flame retardant, have been desired.
Typical examples of non-halogen flame retardants include metal hydroxides such as aluminum hydroxide and magnesium hydroxide. However, in order to pass the Vertical-Specimen-Flame test VW-1 specified in UL standards using a metal hydroxide-based flame retardant, it is necessary to add the metal hydroxide-based flame retardant in a large amount. The addition of the metal hydroxide-based flame retardant in such an amount that the flame test can be passed causes a problem of a significant decrease in the tensile strength and the elongation.
Besides metal hydroxides, organic phosphorus-based flame retardants such as phosphoric acid esters, and nitrogen-based flame retardants are known as the non-halogen flame retardants. However, the flame-retardant effect of these non-halogen flame retardants is not as high as that of halogen-based flame retardants. Accordingly, in reality, satisfactory flame retardancy is not obtained unless such a non-halogen flame retardant is added in a large amount, or the flame retardancy is not improved even if such a non-halogen flame retardant is added in a large amount.
In order to reduce the content of a flame retardant, non-halogen flame-retardant materials containing a flame-retardant resin as a base polymer have been developed.
For example, Japanese Unexamined Patent Application Publication No. 2009-249552 (PTL 3) discloses a cross-linked insulating coating produced by using a resin composition containing a mixture of a polyphenylene ether-based resin and a styrene-based elastomer as a base polymer, a phosphorus-based flame retardant, a nitrogen-based flame retardant, and a multifunctional monomer. In this resin composition, an ester or ammonium salt of condensed phosphoric acid is used as the phosphorus-based flame retardant.
In the resin composition disclosed in PTL 3, a flame-retardant polyphenylene ether-based resin is used instead of a polyolefin-based resin. Therefore, even when the content of the flame retardant is reduced, the resin composition can pass the flame test and satisfy the requirements of the mechanical strength (tensile properties) at room temperature. However, regarding a heat-aging test, an insulating coating merely satisfies a retention of 75% or more after being left to stand at 136° C. for seven days. Thus, the insulating coating cannot be used as an electric wire that satisfies the 150° C. rating.
Japanese Unexamined Patent Application Publication No. 2010-118207 (PTL 4) discloses a cross-linked insulated electric wire including an insulating coating having a two-layer structure. The cross-linked insulated electric wire includes a conductor, a first insulating layer (inner layer) formed on the conductor, and a second insulating layer (outer layer) formed on the first insulating layer. The first insulating layer is composed of a resin composition containing, as a base polymer, a resin prepared by mixing a styrene-based elastomer, a polyolefin-based resin, and a polyphenylene ether-based resin, and, as flame retardants, a phosphorus-based flame retardant (a condensed phosphoric acid ester such as triphenyl phosphate) and a nitrogen-based flame retardant. The second insulting layer is composed of a resin composition containing a metal hydroxide as a flame retardant.
Examples of PTL 4 disclose an insulated electric wire including an inner layer composed of a cross-linked product of a resin composition containing, as a base polymer, a polyphenylene ether-based resin and a styrene-based elastomer, and a condensed phosphoric acid ester and melamine cyanurate, and an outer layer composed of a cross-linked product of a resin composition prepared by adding 180% by mass of magnesium hydroxide to a base polymer. Regarding heat-aging resistance of this insulated electric wire, although the 125° C. rating (heat-aging test at 158° C. for seven days) is satisfied, in the 150° C. rating (heat-aging test at 180° C. for seven days), the retention is about 20% to 30%. Thus, the insulated electric wire cannot be used in applications of the 150° C. rating.    PTL 1: Japanese Unexamined Patent Application Publication No. 5-81930    PTL 2: Japanese Unexamined Patent Application Publication No. 10-168248    PTL 3: Japanese Unexamined Patent Application Publication No. 2009-249552    PTL 4: Japanese Unexamined Patent Application Publication No. 2010-118207